A structure of a conventional acousto optic element is disclosed in Japanese Laid Open Application No: H6-247840 for instance. Following, the disclosed example is explained using FIG. 11 and FIG. 12.
FIG. 11 is a perspective view of a conventional acousto optic element, and FIG. 12 is a perspective view of the acousto optic element cut out along a center line of a piezoelectric transducer in a longitudinal direction, explaining a working principle of the acousto optic element.
The acousto optic element is composed of piezoelectric transducer 110 having a piezoelectric characteristic, and jointed to acousto optic medium 111 at transducer joining plane 116 as shown in FIG. 11. Front side electrode layer 113 and back side electrode layer 114 are formed on each surface of piezoelectric material 112 for applying an electric signal to piezoelectric transducer 110. Acousto optic medium 111 is made in a shape where inclined plane 115 is two-dimensionally tilted as will be described next, and facing toward transducer joining plane 116 to which piezoelectric transducer 110 is attached. In other words, inclined plane 115 is tilted to plane (angle determining plane) 117 of acousto optic medium 111 by an angle θ2 slanted toward a ridge as a reference line to which angle determining plane 117 and light outgoing plane 119 come into contact, as well as tilted to light incoming plane 118 a side plane of acousto optic medium 111 by an angle θ1 slanted toward a ridge as a reference line to which light incoming line 118 and a plane facing angle determining plane 117 come into contact. With this angle of inclination, interference between the ultrasonic wave propagating in acousto optic medium 111 and refracted by inclined plane 115 and the ultrasonic wave newly generated by piezoelectric transducer 110 is controlled.
The working principle of the above mentioned acousto optic element will be explained using FIG. 12. An electric signal emitted by high frequency output source 125 is applied though lead wires 123 and 124 to front side electrode layer 113 and backside electrode layer 114 formed on both sides of surface of piezoelectric material 112, exciting piezoelectric transducer 110. The excited piezoelectric wave is emitted as ultrasonic wave flux 126 to acousto optic medium 111. A thin and thick state is thereby caused inside acousto optic medium 111 in a frequency of ultrasonic wave flux 126, having acousto optic medium 111 works as a diffraction grating. Hence, when incoming light 120 is input to light incoming plane 118 in an appropriate direction, diffracted light 121 is output. However, when the electric signal from high frequency power source 125 is turned off, ultrasonic waves flux 126 from piezoelectric transducer 110 disappears, making the optical media ineffective as the diffraction granting. Diffracted light 121 fades away but through-light beam 122 is output from acousto optic medium 111. Thus, since diffracted light 121 and through-light beam 122 are given out of the optical media when high frequency power source 125 is turned on and off, it becomes possible to modulate the incoming light to the outgoing light.
However, in the conventional acousto optic element, diffracted light 121 is still observed immediately after the electric signal from high frequency power source 125 is turned off. The reason is as follows. Piezoelectric transducer 110 excited by an electric signal generates an ultrasonic wave in a surface leakage mode, propagating on a surface of acousto optic medium 111. And, even after the electric signal is turned off extinguishing ultrasonic wave flux 126, the ultrasonic wave in the surface leakage mode comes back to piezoelectric transducer 110 without fading, exciting piezoelectric transducer 110. Thus, the excitation causes and transmits an unnecessary ultrasonic wave to acousto optic medium 111, diffracting an incoming light.
Putting an intensity of diffracted light generated by an applied electric signal P1, and an intensity of diffracted light caused by an unnecessary ultrasonic wave when the electric signal is turned off P2, a fading ratio of light is expressed by a following formula.Fading ratio of light=10·log(P2/P1)
The fading ratio of light is an important parameter representing performance of an acousto optic element in controlling on/off of a diffracted light and controlling strength of modulation of the diffracted light.
Dropping of the fading ratio of light by the unnecessary ultrasonic wave causing degradation of a picture quality has become a large task of an acousto optic element for optical instrument which is required to draw an image and a letter precisely.
This invention was made in considering above problems, and aiming to provide an acousto optic element having a high light-fading ratio which makes an influence of unnecessary ultrasonic wave small, diffracts an incoming light securely according to a control electric signal, and controls an intensity of the diffracted light.